1. Field of the Invention
The present invention relates generally to electric submersible pump connections. More specifically, the present invention relates to a splined connection for connecting submersible pumps that is removable.
2. Description of the Related Art
It is sometimes advantageous to couple multiple pumps to one motor in an electric submersible pump system. To do so shafts must pass from the motor through one pump to the next and continue to any successive pumps. Therefore, in a two pump system the lower pump drive shaft is connected to the motor at one end, extends through the lower pump and is connected to the second pump at the other end. These pump assemblies can be quite long, with a length to diameter ration greater than 250 being not uncommon.
In order to transmit the high torque between the motor and the pumps, a relatively large diameter coupling is preferred. Such couplings are typically a splined connection wherein each of the ends have matching splines. Normally in a one or two pump assembly, the shaft has a constant diameter, and the grooves for the splines are cut into the shaft at each end. A coupling sleeve having internal splines slides over the upper end of a lower shaft and the lower end of an upper shaft.
Where the second pump connects to the first, less torque is being transmitted (approximately half as much) so conventionally-sized splined ends may be used. Having the shaft ends at the same diameter as the shaft allows the pump components to be assembled and disassembled over the splined ends. The pumps are typically assembled by sliding impellers and diffusers over one end of the pump shaft, each impeller and diffuser having a central bore that closely receives the shaft.
A disadvantage of having the splined ends at the same diameter as the shaft is that the splines form a weak point in the shaft because the shaft has less cross-sectional area at the splined ends due to the splines. The amount of torque that a conventional splined end can handle is significantly less than if the splined end had the same cross-sectional area as the remaining portions of the shaft. The ability to transmit torque is related to the cube of the diameter, so that small decreases in the diameter of splined ends due to the spline grooves mean that the shaft can transmit much less torque. This is typically not a problem in two pump systems because the torque being transmitted from the first pump to the second is about half the torque being transmitted from the motor to the first pump.
For three or more pump systems, one approach is to utilize larger diameter shafts in the motor and each of the pumps. A larger shaft is not a particular disadvantage for the motor. However, larger shafts for the pumps results in less radial distance between the inner and outer portions of the flow channels of the impellers and diffusers, unless the housings are also made larger. It may not be possible to increase the diameters of the housings. Consequently, the pumps with larger shafts may not be as efficient as pumps with smaller diameter shafts.
Another approach is to provide an enlarged end on the lower end of the shaft of the lower pump so that it has the same strength as the remaining portion of the shaft. One prior art technique involves welding or brazing a shaft sleeve, also called a knob spline, onto the end of the shaft. One of the problems with this approach is that the process is very sensitive and the welding requires skilled welders and machinists. Also, this technique results in a permanently enlarged end. However, having a permanently enlarged end does not create a problem if it is only on one end of the shaft because the impellers and diffusers could be assembled over the other end of the shaft.
When more than two pumps are used the torque passed from the first pump to the second pump increases. For example, when five pumps are used the torque being transmitted from the first pump to the second pump is approximately 80% of the torque being transmitted from the motor to the first pump. Because of this, a conventional splined end connection may not be an option between the first and second pump. Both ends of the shaft of the first pump can""t be permanently enlarged because the impellers and diffusers will not slide over a permanently enlarged end. This either limits design flexibility or requires a work around.
A shaft sleeve is attached to a downhole pump drive shaft with a high strength key to transmit high torque loads from one pump to another. Because the shaft sleeve is attached with a key, it is easily removable. The shaft sleeve has exterior splines and effectively increases the diameter of the connection so that larger loads may be transmitted. The shaft sleeve and key can each be manufactured from a variety of high strength materials to deliver the most cost effective coupling for the application.
The shaft sleeve is secured to the shaft end by a threaded section. Preferably, the threaded section is a threaded hole formed in the end face of the shaft end. A fastener with a head secures to the threaded hole. The head of the fastener engages a shoulder on the shaft sleeve to secure it to the shaft end against axial movement. Preferably, a shaft sleeve is mounted to both ends of the shaft of the first pump and to the lower end of the shaft of the second pump. A splined coupling sleeve slides over the shaft sleeves of the mating shaft ends, engaging the external splines to transmit torque from the shaft of the first pump to the shaft of the second pump. The shaft end assembly of the second pump transfers axial down thrust through the head of the fastener to the shaft of the first pump.